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Abstract

The Lower Cretaceous oil sands in the Primrose-Kirby area occur in two superimposed, and geologically very different, reservoir units. The lower reservoir unit is the McMurray Formation, with facies changing from a fully terrestrial, fluvial environment at the base to a marginally marine, deltaic environment with major channels near the top. The Wabiskaw Member of the Clearwater Formation forms the upper reservoir unit, with laterally continuous marine sheet sands. The contact between the McMurray Formation and the Wabiskaw Member marks the transgression of the area, generally from the north, by the boreal sea.

The two reservoirs have very different characteristics, as was revealed by detailed sedimentological, mineralogical, biostratigraphical, and particle size analyses. The channel sands in the McMurray Formation form the most favorable reservoirs because the unconsolidated sands are relatively coarse-grained and well-sorted homogeneous quartzarenites to sublitharenites with no significant amounts of clay minerals or glauconite. The channels are 30-40 m (100-130 ft) thick and have an average bitumen content of 11. 2 wt %. The Wabiskaw sediments are finer grained and more poorly sorted heterogeneous litharenites containing significant amounts of fines. In addition, numerous carbonate-cemented, totally indurated zones, up to 1 m (3 ft) thick, occur. The average bitumen content is relatively low (9. 5 wt %), but the very continuous and predictable thickness of approximately 20 m (65 ft) makes the Wabiskaw a very attractive reservoir.

Both McMurray channel sands and Wabiskaw marine sheet sands with a combined total of 700 million barrels of exploitable bitumen in place in the study area are being tested for performance. The promising preliminary results of both reservoir units indicate that the general Primrose area may provide a significant amount of Alberta’s future oil production.

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